There exists in telephone network technology a continual need to increase the available bandwidth. Traffic on the World Wide Web and similar networks is growing rapidly, and new networks are developing. A number of photonic solutions have been offered that increase the available bandwidth of these networks. These solutions range from point to point connections, to wavelength division multiplexed passive optical network systems. The latter solution is effective in principle, however the cost associated with photonic devices in these systems has been an impediment to their acceptance and rapid deployment.
Recent proposals for wavelength multiplexed passive systems utilize an optical modulator at the subscriber location to replace the active LED or laser devices proposed in prior art systems. See, for example, L. Altwegg, A Azizi, P. Vogel, Y. Wang, and P. Wiler, "LOCNET--a fiber-in-the-loop system with no light-source at the subscriber end", J. of Lightwave Tech., vol. 12, no. 3, pp. 535-540,1994; also see: N. J. Prigo, P. D. Magill, T. E. Darcie, P. P. lannone, M. M. Downs, B. N. Desai, U. Koren, T. L. Koch, C Dragone, and H. M. Presby, "RITE-Net: A passive optical network architecture based on the remote interrogation of terminal equipment," Proc. of the Optical Fiber Conference--post deadline session, (San Jose, Calif., Feb. 20-25, 1994) pp. 43-47. This approach has several advantages, among them lower cost and higher reliability. The passive devices are also less sensitive to temperature variations, and have a robustness suitable for the uncontrolled environment at some customer locations. Additionally, wavelength routing in the network is more reliable since the upstream light is identically the same wavelength as the downstream light.
Recently, a low-cost silicon optical modulator has been developed based on micro electro mechanical systems principles (MEMS) which fulfills the foregoing objectives, e.g. a low-cost, high production volume, modulator. The device has been designated MARS, for Moving Anti-Reflection Switch. In a preferred form, this device has a multi-layer film stack of polysilicon/silicon nitride/polysilicon, wherein the polysilicon is doped and comprises the electrode material. A precisely controlled air gap between the film stack and the substrate allows switching from a reflecting state to an anti-reflecting state. The multi-layer film configuration has been found to give low drive voltage, large optically active area, and good spectral characteristics. It performs well in many system applications but in certain relatively uncontrolled electrical environments, as in some telephone applications, the potential for catastrophic failure arises due to spurious voltage spikes or large changes in the dielectric properties of the air gap. Failure results when the optically active membrane flexes too far and the lower polysilicon metallization shorts to the silicon substrate.